764 



A TEXTBOOK OF THEORETICAL BOTANY 



The outer covering becomes finally orange red in colour, and the middle 

 covering hard and sclerotic with a ridged surface (Fig. 764). 



Remains of 

 micropylar tube 



Perianth 



Outer integument 



Inner integument 

 Nucellus 



Embryo sac 



I 



Fig. 764. — Gnetum sp. Longitudinal 

 section through a mature ovule. 



The Male Gametophyte. 



The male gametophyte is incompletely known. The ripe pollen grain 

 contains three nuclei, one vegetative, and comparable perhaps to the tube- 

 nucleus in other types, the other two being gametic nuclei which enter 

 the embryo sac. There do not seem to be any prothallial cells, but the first 

 nucleus divides into two, one of which again divides and organizes stalk and 

 body cells. The latter divides to form the two gametic nuclei (Fig. 765). 



The Female Gametophyte. 



The female gametophyte appears to differ markedly in difTerent species. 

 In the incompletely investigated but apparently primitive G. iile there seems 

 to be a considerable development of female prothallial cells, occupying about 

 two-thirds of the megaspore, the upper third being filled with a loose collec- 

 tion of tubular, multinucleate cells, which may be parthenogenetic embryos 

 (no pollen tubes having been seen) or perhaps incipient prothallial tubes 

 like those found in Wehvitschia. In G. gnemoji, which is better known, 

 fertilization occurs in the free-nucleate stage of megaspore development. 

 A prothallial tissue develops at the lower (antipodal) end of the megaspore, 

 the cells of which are frequently multinucleate. Below the prothallus, a so- 

 called " pavement tissue " of nutritive cells develops in the nucellus. The 



